1. Field of the Invention
This invention relates to a test bench for total or partial wideband radio links.
Radio link test benches, also referred as to radio link analyzers, are used to determine the transfer function of all or part of the radio link. The transfer function is designed by variations in amplitude and the group propagation time depending on the frequency. The wideband measurement techniques, i.e, for a bandwidth greater than 100 MHz, apply to wideband radio transmission equipment such as those included in the land-based transmission systems or earth-satellite transmission systems, or their subassemblies, such as waveguides or antennae, common to the whole communication passband, and also to the propagation environment.
2. Description of the Prior Art
In a radio link test bench, a frequency variation is derived over each frequency within the frequency band being explored to obtain, on receiving, a corresponding phase variation in the received signal. From this phase variation it is possible to determine the group propagation time equal to the derivative of the phase depending on the frequency.
Such a test bench includes a transmission section and a receiving section. On transmission, a radio frequency wobbulator receives on input the sum of low-frequency signal controlling the exploration of the frequency band of the link and a measurement signal at intermediate frequency to supply on output, a radio frequency signal wobbulated by the input signal. The wobbulated radio frequency signal is transmitted by the wideband radio link. On receiving, the received wobbulated radio frequency signal is reduced to a lower frequency by a frequency down-converter and is then demodulated in a radio link analyzer so as to obtain the group propagation time information. The amplitude information is deduced by simply detecting the amplitude of the wobbulated signal on output from the frequency down-converter.
In order to determine the transfer function of the radio link on receive, it is necessary to know at each moment the frequency of the wobbulated radio frequency signal received. It would therefore appear that the frequency information should be measured in the receiving section or that it should be transmitted as from the transmission section, for example via pulses carried by the wobbulated radio frequency signal emitted. Each pulse corresponds to exploration passing through certain predetermined frequencies of the band. Owing to the nonlinearity and unstability of the frequency compression, the receiving measurement on output from the down-converter is subject to error. The signal can also be disturbed by the propagation environment examined or by the transmission equipment.
Two techniques employed to identify frequencies have been described in the paper by Robert EASSON and Robin SHARP entitled "AMELIORATION DES MESURES SUR LES EQUIPMENTS FAISCEAU HERTZIEN AU MOYEN D'UN CONVERTISSEUR-ABAISSEUR A POURSUITE" ("Improvement to measurements on radio links by means of a tracking down-converter") read at the "SYMPOSIUM sur les TELECOMMUNICATIONS et la TELEINFORMATIQUE" on Mar. 14 and 15, 1986, Palais des Congres, Versailles, France.
A first known technique consists in producing, on receiving, a marking signal for one frequency. The high frequency signal taken on receiving, via a directional coupler, is sent to a cavity type wavemeter followed by a detector, to obtain a synchronization pulse when passing through the tuning frequency of the cavity. This technique however offers the drawback of requiring a cavity that can be tuned by a frequency in the band explored, to be identified. In this technique, as the pulse amplitude depends on the level of received signal, the frequency information is liable to be indiscernable from the signal amplitude variations, which may entail errors in identifying the frequencies.
Another technique consists in modulating, on transmission, the amplitude of the radio frequency signal, by pulses at times corresponding to the exploration passing through predetermined frequencies of the explored band. On receiving, a series of marking pulses is obtained and is added to the amplitude information signal. With this technique it is not possible to make a correct separation between the frequency information and the amplitude information. In fact, either the amplitude modulation is too weak and the pulses intermingle in the amplitude variations, and it is impossible to discern them, or else the amplitude modulation is too strong and the receiver receives an insufficient signal. Moreover, the measurement of the amplitude variations is erroneous owing to the presence of the marking pulses.
The accuracy obtained by these known techniques is low and generally limited by the width of the marking pulses or by the nonlinearity of frequency-compression receiving. The relative error obtained for an excursion of around 1 GHz is about 5%.